1. Field of the Invention
This invention relates to an X-ray lithography mask and a method for manufacturing the same, and more particularly to an X-ray lithography mask which is suited for use in the manufacture of a semiconductor device such as a very large scale integrated circuit (VLSI) having a submicron pattern.
2. Description of the prior art
Before X-ray lithography came to be widely used, the lithography technology resorted to was the so-called photolithography which employed an ultra-violet ray emitted from a high pressure mercury vapor lamp and the like. Due to the fact that a minute pattern, on the order of a submicron is often desired, photolithography can no longer be realistically resorted to because of the diffraction effect and the diffusion effect of an ultra-violet ray in the photo-resist. The lithography technique which is now used instead of this outdated technology is the so-called x-ray lithography which utilizes rays of a shorter wave length than ultra-violet rays.
In the case of X-ray lithography, an X-ray lithography mask, adapted to employ a shadow printing technique similar to the photolithographic technique, is placed between an X-ray source and an object to be exposed. X-ray flux is then irradiated over the entire area of the mask and an X-ray-sensitive material film, namely an X-ray resist film, formed on the object, is thereby selectively exposed to the X-ray. In this manner a submicron pattern, formed on the mask, can be transferred to the object. An X-ray provides greater penetrating power to a material than does an electron or photon beam and hence is not susceptible to scattering or reflection depending on the kinds of materials used. Therefore, X-ray lithography allows an increase in the resist thickness while retaining desired resolution. This leads to an improvement in reliability of an etching mask in a subsequent etching process. A description of fundamental X-ray lithography techniques is given in U.S. Pat. No. 3,743,842 and in the "Proceedings Of The IEEE" Vol. 62, No. 10, Oct. 1974, pages 1361 to 1387.
X-ray lithography is an effective process for manufacturing a semiconductor device, and especially for manufacturing VLSI VLIC having a submicron pattern. However, the following requirements must be met when using the X-ray lithography technique:
(i): A mask alignment must be made accurately and easily. PA1 (ii): The mask membrane on which the mask pattern is formed, must be strong and the mask area, in other words the membrane area, must be large.
In general, in the manufacture of semiconductor devices, a set of at least four to eight lithography masks having different patterns are used. An important step in the manufacture of the semiconductor devices is so-called mask alignment, in which one pattern of a lithography mask is laid on another pattern of a lithography mask, while retaining a given positional relationship. In addition to the accuracy of the lithography mask itself, accuracy, operability of alignment, and product yield have great influence on the success of manufacturing semiconductor devices such as VLSI's.
The finer the pattern to be transferred, the higher alignment accuracy required. The higher alignment accuracy required, the lower operability and yield of product. The alignment accuracy required in general is on the order of less than 1/10 of the minimum line width of a pattern. For instance, in a high quality VLSI an alignment accuracy of at least .+-.0.1 to 0.3 .mu.m is required.
In the prior art X-ray lithography mask disclosed in U.S. Pat. No. 3,742,230, a X-ray transmissive layer, namely a membrane, carrying a X-ray absorbing layer, constituting a submicron pattern, is made of a silicon film that is opaque to visible lights. This X-ray transmissive layer is supported by a silicon rib and doped with an impurity element having a smaller covalent bond radius than silicon for imparting a tension to the X-ray transmissive layer. The silicon rib is doped with an impurity element having a covalent bond radius larger than silicon. The critical disadvantage in this X-ray lithography mask is that the mask is opaque to visible light and hence mask alignment using visible light or laser beam is impossible.
Another prior art mask is disclosed in U.S. Pat. No. 3,742,229. This mask utilizes transmissive X-rays to align the mask in an accurate position on an object such as semiconductor substrate. This X-ray lithography method however is not practical for manufacturing semiconductor devices. This is due to the fact that heavy metal such as gold, formed on the semiconductor substrate as an alignment pattern degrades the performance of the semiconductor device when the substrate is subjected to high temperature processes such as a CVD process subsequent to the X-ray lithography process. Moreover, a portion of the substrate on which the gold pattern is formed must be so thin so as to transmit the X-rays for alignment. Therefore treatment of the substrate becomes difficult.
Another prior art X-ray lithography mask is proposed in "SOLID STATE TECHNOLOGY," Sept. 1976, pages 55 to 58, and in the "IBM TECHNICAL DISCLOSURE BULLETIN," May, 1976, No. 12, Vol. 18, pages 4210 to 4211 by E. Bussous et al. According to this method, the membrane is formed by laying a silicon oxide on a silicon nitride layer, and then chromium and gold double layers are laid on the silicon oxide layers. In this case, the chromium and gold double layer are extremely thin, so that they are somewhat transparent to visible light. However, this structure is low strength, and hence fails to provide a large pattern.
It is therefore an object of the present invention to provide an X-ray lithography mask, which enables accurate and easy mask alignment.
It is a further object of the present invention to provide a large pattern X-ray lithography mask.
It is another object of the present invention to provide a method for manufacturing an effective X-ray lithography mask with conventional semiconductor device processing methods.